Options for Checking Septic Tank Watertightness

Options for Checking Septic Tank Watertightness

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There are many reasons to ensure that all septic tanks are watertight. Leakage from the tank releases minimally treated sewage into subsurface soils and/or groundwater. Sewage injected deeply in the soil profile is much less likely to be adequately treated as it moves down through the soil. In areas of relatively shallow water tables or where tanks are located in low areas, groundwater or surface water can leak into the tank. Inflow of groundwater can disrupt settling, treatment and storage of solids (i.e., the important functions of the tank), as well as the function of downstream components of the wastewater treatment system.

Possible locations on a septic tank where leakage can occur include:

  • Weep holes at the base of the tank. Weep holes are used in some precast concrete tanks to release forms from tanks and to prevent collection of rainwater during storage prior to installation. These are best avoided, but if used, they must be sealed appropriately prior to installation. They are not allowed in many state/local codes.
  • Midseam joint
  • Inlet/outlet pipe penetrations
  • Top-seam joint
  • Tank top/access riser joint
  • Access riser/lid joint
  • Any damaged, improperly formed location or area where material is too thin.

A new tank can be tested for watertightness by filling it with water (hydrostatic testing) or by vacuum testing. In both cases, the tank should be tested in the ready-to-use state. Inlets and outlets should be plumbed with the appropriate pipes, which can then be plugged for the test.  

Hydrostatic testing

Be careful when performing hydrostatic tests on plastic and fiberglass tanks, as they gather much of their strength from the soil support. For all midseam tanks, keep the backfill near the midseam but leave the seam itself exposed to monitor the test. 

The following is a suggested water testing procedure for tanks. Note that this test does not evaluate the tank’s ability to withstand external pressures; that issue must be ensured through adequate engineering design. 

  1. Plug the inlet and outlet pipes with a watertight plug, pipe and cap, or other seal. Seal the pipes away from the tank to test any pipe connections that may be of concern.
  2. If testing a midseam tank, ensure the seam is exposed for the water test.
  3. Fill the tank to the top.
  4. If the tank has a riser, add water into the riser to a maximum of 2 inches above the tank-riser seam. Care must be taken not to overfill, as the top section of a two-piece tank may become buoyant. 
  5. Measure and record the water level.
  6. Wait 24 hours. Any obvious leakage during this time should be evaluated and remedied by the application of a suitable sealing compound. 
  7. If the test reveals leaks that cannot be repaired, the tank is considered unacceptable.
  8. Refill concrete tanks to original level after 24 hours, as they will absorb some water.
  9. Check again after 24 hours. If less than 1 gallon is lost in a concrete tank, the leak test is considered acceptable.  

When performing hydrostatic testing in cold climates, there are a few important points to consider. First, water is its densest at about 39 degrees F; water put into a tank at 50 to 60 degrees F (typical of groundwater) and left in the tank overnight at freezing temperatures will drop the level in the tank a substantial amount (about 2%, or 3 gallons in a 1,500-gallon tank). A loss of 3 gallons in the risers will look like a leak. Additionally, water used in the test will freeze and expand by approximately 9%. If the site is not occupied quickly, the tank may crack as a result of the test itself, assuming the water is left in the tank following the test.

Vacuum testing

Vacuum testing of tanks requires less time than hydrostatic testing and can be performed without having water available on the site. Testing should be done on the tank in its ready-to-use state (i.e., pipes in the inlet and outlet, risers with lids.) In this test, all pipe penetrations, manholes and risers are sealed airtight, and a special insert is sealed on one of the tank manholes. Using a pump, air is evacuated through this insert to a standard vacuum level, and the reading on a vacuum gauge is recorded. Local codes, ASTM C1227 or the National Precast Concrete Association standard can be used to determine the target vacuum for the size, shape and tank material being used. Please remember that different tank construction materials (e.g., concrete, plastic and fiberglass) have different compressive strengths. Be careful not to exceed the recommended vacuum level for a tank as recommended by the manufacturer. It is possible to damage or implode a tank.

The NPCA standard states: “The recommended vacuum test procedure is to introduce a vacuum of 4 inches of mercury. Hold this pressure for five minutes. During this initial five minutes, there is an allowable pressure equalization loss of up to 1/2 inch of mercury. If the pressure drops, it must be brought back to 4 inches and held for a further five minutes with no pressure drop.”

If a tank will not hold the vacuum, leaks must be located and repaired. The test can then be repeated. If the tank cannot be repaired and rendered watertight, it should be replaced. Note that vacuum testing of concrete tanks draws seams together for a positive mastic seal, assuming there are no other problems. With any tank, collapse, deflection, deformation or cracking indicate a poor-quality tank. It is important to test the entire system: tank, pipe sleeves, risers, inspection ports and lids.

About the author: Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.


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